Classes of Plywood
Two classes of plywood are commonly available, covered
by separate standards: (a) construction and industrial ply-
wood and (b) hardwood and decorative plywood.
Most construction and industrial plywood used in the United
States is produced domestically, and U.S. manufacturers
export some material. The bulk of construction and indus-
trial plywood is used where performance is more important
than appearance. However, some grades of construction and
industrial plywood are made with faces selected primarily
for appearance and are used either with clear natural finishes
or lightly pigmented finishes. Construction and industrial
plywood has traditionally been made from softwoods such
as Douglas-fir and southern yellow pine. However, true firs,
western hemlock, and western pines are also used (Bowyer
and others 2007). A large number of hardwoods qualify for
use under the standard. PF resin is the primary adhesive type
used in construction and industrial plywood. Construction
and industrial plywood is categorized by exposure capability
and grade using Voluntary Product Standard PS 1–07 (NIST
2007).
Hardwood and decorative plywood is made of many dif-
ferent species, both in the United States and overseas. Well
over half of all panels used in the United States are im-
ported. Hardwood plywood is normally used in applications
including decorative wall panels and furniture and cabinet
panels where appearance is more important than strength.
Most of the production is intended for interior or protected
uses, although a very small proportion is made with ad-
hesives suitable for exterior service, such as in marine ap-
plications. A substantial portion of all hardwood plywood
is available completely finished. Hardwood and decorative
plywood is categorized by species and characteristics of
face veneer, bond durability, and composition of center
layers (veneer, lumber, particleboard, MDF, or hardboard)
(HP–1–2004, HPVA 2004).
Exposure Capability
Construction and industrial plywood is classified as either
Exposure 1 or Exterior in Voluntary Product Standard PS
1–07 (NIST 2007). Exposure 1 plywood is intended for
applications not permanently exposed to weather, whereas
Exterior plywood is suitable for repeated wetting and dry-
ing, or long-term exposure to weather. Bond quality of ply-
wood of either bond classification (Exposure 1 or Exterior)
is evaluated by the same test procedure, but a higher level
of performance in the test procedure is required for Exterior
plywood. The test procedure involves water saturation, boil-
ing, and high-temperature exposure (in excess of boiling
temperature). The majority of construction and industrial
plywood sold in North America is of Exposure 1 classifica-
tion. Exposure 1 panels may undergo rain-wetting during
building construction but will be protected from wetting
after the building is enclosed.
Two exposure classes of hardwood and decorative plywood
are recognized by ANSI/HPVA HP–1–2004, Exterior and
Interior. The standard actually lists two different Exterior
classes, Technical and Type I, but the bond performance
requirements for these classes, as determined by test proce-
dures outlined in the standard, are the same.
Plywood Grades
Plywood grades may indicate the intended use, a type of
surface treatment, or the grades of the face and back ve-
neers, and in some cases, a combination of these. Agencies
that provide quality certification services for plywood mills
have coined their own trademarked grade names for speci-
fied end uses through proprietary product standards. Grade
stamps are used to identify plywood products (Figs. 11–6
and 11–7). An example of plywood CARB third-party iden-
tification is also shown in Figure 11–7.
Veneer quality is a factor in construction and industrial ply-
wood based on visually observable characteristics. Knots,
decay, splits, insect holes, surface roughness, number of
surface repairs, and other defects are considered. Veneer
species and characteristics are also a major factor in catego-
rization of hardwood and decorative plywood.
Specialty Plywood Panels
Plywood is easily pressure-treated with waterborne preser-
vatives and fire retardants. Because plywood is not prone
to irreversible thickness swelling, its bond integrity is unaf-
fected by pressure treatment with waterborne chemicals.
Treatment is typically performed by commercial entities
specializing in treatment rather than by the plywood manu-
facturer. Treatments for plywood have been standardized
(AWPA 2007a,b). This allows specification by reference to
a commercial standard. Special grades of plywood are pro-
duced for specific uses such as boat construction, concrete
form work, or special exterior applications such as highway
signage.Oriented Strandboard
Oriented strandboard (OSB) is an engineered structural-use
panel manufactured from thin wood strands bonded together
with water-resistant resin, typically PF or pMDI. It is used
extensively for roof, wall, and floor sheathing in residential
and commercial construction. The wood strands typically
have an aspect ratio (strand length divided by width) of at
least 3. OSB panels are usually made up of three layers of
strands, the outer faces having longer strands aligned in the
long-direction of the panel and a core layer that is counter-
aligned or laid randomly using the smaller strands or fines.
The orientation of different layers of aligned strands gives
OSB its unique characteristics, including greater bending
strength and stiffness in the oriented or aligned direction.
Control of strand size, orientation, and layered construction
allows OSB to be engineered to suit different uses.
OSB technology and the raw material used originally
evolved from waferboard technology, for which aspen wasChapter 11 Wood-Based Composite Materials